The present invention relates to a method and an apparatus for controlling a scintillation camera and, more particularly, to a method and an apparatus for effectively controlling the field uniformity of a scintillation camera with efficiency and precision.
A scintillation camera is used for detecting radiation (usually gamma rays) emitted by a body to whom small amounts of radioisotopes have been administered. The radiation emitted by the tissue of the body under examination is guided to a scintillator by a collimator in such a manner that the point of the body emitting the radiation coincides with the point at which the radiation is absorbed by the scintillator.
The scintillator absorbs the radiation to cause scintillation and to convert the radiation into visible light. A plurality of photomultiplier tubes are optically coupled to the scintillator to convert the visible light into electric signals. These electric signals are supplied to a position calculating circuit through a waveform shaping circuit. The position calculating circuit calculates the X- and Y-coordinates of the point of the body emitting the radiation and supplies X- and Y-coordinate signals obtained to a cathode ray tube. Based on the X- and Y-coordinates thus supplied and an unblanking signal, the cathode ray tube screen displays the distribution of the radioisotopes present in the tissue of the body under examination.
In general, in a scintillation camera adopting a resistor matrix for the position calculating circuit, radiation detection pulse signals supplied from the photomultiplier tubes are subjected to a nonlinearity correction at the waveform shaping circuit to correct the field nonuniformity of the display on the cathode ray tube. However, due to variations in the characteristics of the optical system of the radiation detector including the scintillator and so on, the responses of the photomultiplier tubes (to be referred to as PMT responses hereinafter) included in this optical system vary to a non-negligible extent. Therefore, the parameters of the nonlinearity of the waveform shaping circuit must be changed for the output of each photomultiplier tube. However, since this method takes a very long time, actually the nonlinearity parameters of the waveform shaping circuit are obtained according to the output of a representative photomultiplier tube selected.
The parameters of the respective waveform shaping circuits have conventionally been determined by obtaining the PMT responses in the following manner. FIG. 1 shows a conventional method for collecting the PMT responses.
A photomultiplier tube 10 is arranged in the vicinity of a scintillator 14 through a light guide 12 for receiving radiation from the scintillator 14. A point source of radiation PS such as .sup.57 Co is manually moved over the surface of the scintillator 14. The output end of the photomultiplier tube 10 is connected through a cable 15 to the input end of a waveform shaping circuit 18 which performs the waveform shaping and the nonlinearity correction and which constitutes part of an analyzer unit 16. Although only one photomultiplier tube 10 and waveform shaping circuit 18 are shown in the figure, they are the same in number as the number of channels. The output of the waveform shaping circuit 18 is usually input to a position calculating circuit of the resistor matrix type. However, it is input to a multi-channel analyzer 20 here to obtain the PMT responses. FIG. 1 shows a case wherein the PMT responses are obtained by a system including the photomultiplier tube 10, the waveform shaping circuit 18, and the optical system. The point source of radiation PS of .sup.57 Co is placed on the surface of the scintillator 14 at the center of sensitivity of the photomultiplier tube 10. The multi-channel analyzer 20 records the peak value of the channel of the waveform at which the scintillations are most frequently received. Thereafter, the point source of radiation PS is sequentially displaced from the center of sensitivity and the same procedure is repeated. FIG. 2 is a characteristic curve obtained in this manner, which represents the PMT responses. Therefore, the PMT response is the distribution characteristic f(x) of the sensitivity as a function of distance x from the center of sensitivity of the photomultiplier tube 10.
A nonlinear circuit incorporated in the waveform shaping circuit 18 is of the circuit configuration as shown in FIG. 3. An input terminal IT1 is connected to the base of a transistor Tr1 through a resistor R1. A voltage of +15 V is applied to the collector of the transistor Tr1, and a voltage of -15 V is applied to the emitter of the transistor Tr1 through a resistor R2. Connected in parallel to the base of the transistor Tr1 are a series circuit of a diode D1 and a resistor R3, and a series circuit of a diode D2 and a resistor R4. An output terminal OT1 is connected to the emitter of the transistor Tr1. The nonlinear circuit is a polygonal line nonlinear circuit wherein the point of discontinuity of the polygonal nonlinear characteristic curve is determined by reference voltages L1 and L2 supplied to the resistors R3 and R4, respectively, of the series circuits described above.
The parameters of the resistors R3 and R4 of this nonlinear circuit are experimentally determined based on the PMT responses under conditions where the nonlinearity correction is not effected with L1 and L2 being large. After the parameters of the circuit are determined in this manner, the reference voltages L1 and L2 are varied, and photographs of the CRT display are taken according to the respective combinations of the reference voltages L1 and L2. These photographs are arranged in the manner as shown in FIG. 4.
FIG. 4 shows the photographs taken according to various combinations of the reference voltages L1 and L2 for examining the uniformity of the image wherein the reference voltage L1 is plotted along the abscissa and the reference voltage L2 is plotted along the ordinate. The group of photographs for examining the uniformity will hereinafter be called a map. Since the region where L1&lt;L2 corresponds to the region wherein L1&gt;L2 except that L1 and L2 are interchanged, the description will only be made with reference to the region wherein L1.ltoreq.L2. Positions in the same neighborhood of the map thus obtained are under similar conditions and show similar uniformity characteristics. Therefore, the reference voltages L1 and L2 with which excellent uniformity is obtained may be readily determined from the good or poor uniformity distribution on the map. The uniformity control is thus completed by setting the reference voltages L1 and L2 at values at which the uniformity is optimum.
However, such a control method has certain drawbacks which are described below:
(1) Since the point source of radiation PS is manually moved, the precision in setting the position poses a problem.
(2) Since the measurement of the PMT responses are performed manually, a relatively lengthy amount of time is required for the measurement.
(3) Since the parameters for the nonlinear correction are experimentally determined, the resultant parameters are not stable and the uniformity condition varies from one scintillation camera to another.
(4) The standards for setting the reference voltages L1 and L2 for obtaining the optimum uniformity in the map are not established.